In the EDM process, electric pulses are passed intermittently across the machining gap formed in a dielectric liquid between a tool electrode and a workpiece, each pulse bringing about an electric discharge through the dielectric-filled gap to remove material from the workpiece. As the material removal or machining proceeds, the tool electrode is advanced relative to the workpiece by a servo feed arragement to maintain the machining gap at a given size. The servo arrangement may also function to respond to gap short-circuit and arcing conditions to retract the electrode relative to the workpiece thereby removing such conditions.
During the maching operation, small chips and particles, tar and other materials produced by electric discharges are carried away by the dielectric liquid. The latter is generally circulated through the gap and forced to flow from or to be sucked into a bore formed through the electrode and/or the workpiece and directly open to the machining region. In certain EDM applications, however, in which machining is to be performed under "no wear" conditions, requiring minimization of the wear of the tool electrode by the machining discharges, it has been found to be undesirable to forcibly circulate the dielectric liquid through the machining region, at least during the time in which a train of machining discharges are produced through the gap. Accordingly, in such applications, it has been the practice to interrupt the supply of machining pulses to the gap periodically and to effect the forced dielectric supply through the tubular electrode or workpiece selectively during the pulse interruption periods. Another technique which has been used heretofore is to intermittently retract or reciprocate the electrode relative to the workpiece independently of the gap adjusting function by the servomechanism. This latter technique, in which the retraction of the electrode relative to the workpiece is followed in each reciprocating cycle by returning them to their normal, machining relative positions, has been found to be an especially simple and advantageous method effective to cleanse the gap contaminants while permitting the machining action to proceed under the required no-wear conditions. In this case, one or more nozzles can be employed and positioned externally of the machining gap to direct a stream or streams of the fresh dielectric liquid toward the machining region.
As machining proceeds and the tool electrode penetrates progressively into the workpiece, increasing the depth of a cavity being formed therein, the discharge condition in the gap tends to become generally unstable, as a result of increasing difficulty in removing chips and other contaminants from the region of the gap cavity, thereby causing undesirable short-circuiting and substained arcing to develop between the tool electrode and the workpiece and damaging either or both of them. In order to counter these difficulties, various corrective and programmed or anticipatory measures have been proposed and are being used in existing EDM machine tools. These measures include changing the rate of dielectric flow through the gap and the use of a reciprocatory movement as mentioned in the preceeding paragraph in a controlled manner. Also, control of machining pulses has been found to be important. For such control, reference is here made to KIYOSHI INOUE U.S. Pats. No. 3,539,755 issued Nov. 10, 1970 and No. 3,536,881 issued Oct. 27, 1970. These patents disclose concepts for adaptive control of the gap current by controlling machining pulse on time and/or off time in response to a signal derived from the gap to optimize the EDM process by protecting the gap against sustained arcing and other undesirable conditions or by maintaining the gap current density constant with the process of machining. It has also been found desirable to alter, in the course of a machining operation, a threshold value settable in a servocontrol circuit and in a machining pulse generator circuit in order that the problem as noted above be removed or alleviated effectively. For this type of control concept, reference is here made to KIYOSHI INOUE U.S. Pat. Nos. 3,604,885 and 3,686,461 issued Sept. 14, 1971 and Aug. 22, 1972, respectively. Proposals have also abeen made in the art heretofore to conrollingly change voltage and current magnitudes of a series of machining pulses or the waveform configuration thereof with the progress of machining or in response to the gap condition.
While, by virtue of these control concepts, significant advances have been made in the art of EDM to improve the quality of machining and it has become possible to automatize an EDM operation, existing systems embodying these control concepts are found not to be satisfactory in terms of machining efficiency which can be expressed as the time required to complete a given machining operation and to have a desired result. Furthermore, many EDM control systems now brought into practice are, in spite of their efforts, not successful to completely prevent or suppress thermal arcing which damages the workpiece or the tool electrode or both as noted previously. If successful, these efforts may result in an undue decrease in machining efficiency. In many commercial EDM machine tools, the occurrence of the arcing damage could only be prevented by the skilled operator's supervision of the machine or otherwise by the stopping of the machining operation. In summary, therefore, there are vital needs for a control system which provides an increased machining efficiency, insures excellent machining results which are free from arcing damages and permits the operation to proceed on a full or practically completely automatic basis.